1. Field of the Invention
The present invention relates to a radiograph read apparatus, more particularly, it relates to a digital X-ray image processing apparatus mainly used in the field of medical equipment.
2. Description of the Related Art
An X-ray apparatus is widely used in various fields, particularly in the medical field. Conventionally, as an X-ray apparatus having high sensitivity and high resolution, there is a digital X-ray image processing apparatus displaying an X-ray image on a cathode-ray tube (CRT) instead of a conventional radiograph.
The above conventional digital X-ray image processing apparatus (see, for example, U.S. Pat. No. 3,859,527) uses a photostimulable phosphor plate constituted by a sheet-like fluorescence medium which is able to accumulate a part of the X-ray energy. This fluorescence medium accumulating the X-ray energy is called an "accumulative fluorescence body". The accumulative fluorescence body can accumulate the X-ray energy for a relatively long time.
In general, the digital X-ray image can be obtained by the following steps. That is, when the X-ray is irradiated on to an object, for example, a human body, the X-ray transmitted through the object exposes the photostimulable phosphor plate. When the photostimulable phosphor plate is scanned by an excitation beam (for example, a laser beam), the energy accumulated on the fluorescence body is excited by the laser beam and a fluorescent light corresponding to the accumulated energy is emitted from the photostimulable phosphor plate.
The fluorescent light is collected by collection equipment, for example, bundled optical fibers, and converted to analog electrical signals by an optical-to-electrical converter. Further, the analog electrical signals are converted to digital signals to obtain a digital X-ray image on a monitor display apparatus.
In this case, to determine conditions in an actual reading of the X-ray image from the photostimulable phosphor plate, it is important to previously read an outline of the X-ray image (i.e., the state of the X-ray energy) accumulated on the fluorescence body prior to the actual read (below, "pre-scan").
The "actual read" means, in this case, that a doctor reads the X-ray image displayed on the monitor when diagnosing the object. For the photographic conditions during the actual read, it is necessary to determine a supply voltage for an X-ray tube, a multiplication rate of a photomultiplier, an amplification rate of the amplifier, and a distance between the X-ray tube and the object. In general, this photographic condition data is determined from pre-scan data.
There are some methods for pre-scanning in the conventional art.
In one method, prior to the actual read, the photostimulable phosphor plate is scanned by an excitation beam which is weaker than the excitation beam for the actual read. The quantity of the fluorescent light emitted by the weak excitation beam is measured by suitable measuring equipment so that it is possible to perform the pre-scan prior to the actual read.
In another method, when the X-ray is irradiated onto the photostimulable phosphor plate, a momentary fluorescent light is generated from the photostimulable phosphor plate. In this method, the momentary fluorescent light is measured by suitable measuring equipment so that it is possible to perform the pre-scan prior to the actual read. This method utilizes the characteristic that the intensity of the momentary fluorescent light is proportional to that of the fluorescent light in the actual read.
However, in these methods, much data which is not necessary for the diagnosis, such as a very strong X-ray intensity range and a very weak X-ray intensity range, are contained in the pre-scan data. That is, for example, in the diagnosis of the breast of a human body, although the intensity range of the X-rays for the diagnosis of the lungs is different from that of the X-rays for the diagnosis of ribs, various intensities of the X-rays are mixed in the pre-scan data.